1. Field of the Invention
The present invention relates generally to semiconductor devices. More particularly, the present invention relates to copper wire bonding for semiconductor devices.
2. Background Art
Conventionally, gold has been the material of choice for wire bonding, as gold is soft enough to bond readily to most integrated circuit (IC) pad surfaces while providing sufficient conductivity. Thus, IC pad surfaces have generally evolved in design to support gold wire bonds. However, with steadily increasing material costs for gold and ever increasing requirements for improved electrical and thermal performance, industry demands for suitable alternative materials are becoming increasingly urgent.
One particularly promising material is copper, which provides similar self-inductance and self-capacitance properties as gold while reducing electrical resistance and improving thermal conductivity. Additionally, the material cost of copper is much less compared to gold. However, due to the increased hardness of copper compared to gold, it is often difficult to provide a secure and reliable copper wire bond connection to IC pad surfaces that are generally designed for gold wire bonds. More specifically, the harder and less pliable copper wire ball bond on the IC pad surface creates issues such as first bond pad metal cracking or peeling and silicon or dielectric cratering and cracking. While IC wafers may be plated with electro-less Ni/Pd/Au for a thicker and more robust metal pad structure suitable for copper wire bonding, such a procedure may be impractical due to cost and limited availability of qualified sources.
Copper wire also presents technical challenges in applications where via arrays are desirable under a bond pad, for example to improve thermal and/or electrical conduction. The use of conventional grid via array designs with vias densely grouped under the copper wire bond at the bond pad creates stress risers. To avoid this issue, it is known to use a perimeter via array design by placing vias only at the perimeter of the metal pad under the passivation. However, the low via density of the perimeter via array compared to the grid via array results in higher resistance and significantly lower performance.
Furthermore, to provide advantages such as higher moisture sensitivity level (MSL) ratings and simplified package assembly cycles, it may be desirable to use leadframes with pre-plated finishes such as Ni/Pd/Au instead of conventional copper leadframes with Ag plating, which cannot reach high MSL ratings and further require a separate plating step. However, since copper wire does not readily bond to such pre-plated leadframes, it may prove difficult to connect ICs to the leadframe using copper wire.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a simple, cost effective, and reliable way to connect copper wire to IC bond pads and pre-plated lead frames.